Recently, data traffic has sharply increased due to the spread of smart phones. According to Korea Broadcasting and Communication Commission, it was reported that the data traffic had increased three times in 2013, compared to the preceding year. Considering that the number of smart phone users will increase more, and application services using the smart phones will be provided more as well, mobile data traffic is expected to increase at a faster rate than it increases now. Particularly, when the machine-to-machine (M2M) communication, which is a new mobile communication market beyond communications between users, in which communication is made between a user and a machine, or between machines, is widely used, the data traffic to be transmitted to base stations will increase too much to be controlled.
Therefore, technologies to solve the problem above are required, and recently, communication between devices has been noticed as one of the technologies. The technology called device-to-device (D2D) communication has been noticed in the field of a licensed band that is used in cellular mobile communication, and an unlicensed band that is used in communication such as a wireless LAN.
The D2D communication may be fused with the cellular mobile communication in order to give beneficial effects by which a traffic accommodation capacity of a base station can be enhanced, and overload can be reduced. That is, user equipments (UE) in the same cell or in the adjacent cells establish a D2D communication link between them and exchange data through the D2D communication link without passing through the base station {or an evolved NodeB (eNB)}, so two links (that is, one link from one UE to the eNB, and the other link from the eNB to the other UE) may be decreased to a single link (that is, the link from one UE to the other UE).
Research for the unlicensed band is aimed at recognizing demand for communication between users, communication between a user and a machine, and communication between machines to prevent a waste of wireless resources and at adequately determining the traffic that is locally generated to thereby provide services. Accordingly, the research is focused on a method for effectively operating the process in which a plurality of devices broadcasts and receives information on the service or the content.
In order to perform D2D communication, devices are required to be synchronized with each other. The device establishes synchronization between devices using time information received through a synchronous base station or a global positioning system (GPS) receiving module. In the method by which the synchronization is made using the time information received through a synchronous base station or a GPS receiving module, the device is required to connect with the synchronous base station or the GPS receiving module.
In some cases, a communication provider may not provide the synchronous base station, depending on the communication type supported by the communication provider. In this case, it is not likely to make synchronization using the time information received from the synchronous base station. In addition, if the device is located in a shadow area of the GPS (e.g., between high-rise buildings, in tunnels, in buildings, or the like), the device may not receive the time information to thereby fail to make synchronization. Consequently, when a connection with the synchronous base station or the GPS receiving module is not stable, the device cannot initiate the D2D communication.